In the blow-molding of containers, the procedure usually followed is to extrude a synthetic-resin thermoplastic parison from an extrusion head into the path of a closable blow mold, to close this mold on the plastically deformable parison, and to introduce air or some other fluid under pressure into the parison to expand the latter against the inner wall of the mold cavity defined by the blow mold to impart the desired shape to the parison. The mold article is then cooled so that its shape is retained, the blow mold is opened and the finished article is removed.
Where large-volume containers are of interest, it is desirable to equip them with the usual fitting for filling and dispensing the contents, e.g. a thickened mouth and/or neck which can be provided with a cover, if desired, and with a handgrip or handle to facilitate manipulation of the container. Such containers may be used for the transportation, storage and dispensing of a variety of liquids, e.g. milk, oil.
It has been proposed heretofore to provide synthetic-resin containers such as canisters by blow-molding processes so that during the blow-molding operation, the handgrip members are fused to the walls of the container. This can be accomplished by prefabricating the grip, laying it into the blow mold and fusing the grip to the thermoplastic material of the parison during the blowing operation. This mode of manufacture is expensive and unreliable because the step of inserting the prefabricated grip into the blowing mold interrupts the continuity of the working cycle. Furthermore, the bond between the prefabricated grip and the container formed by blowing is frequently insecure since it depends upon the nature of the materials constituting the prefabricated grip and the parison, upon variations in thickness and hence the prefabrication techniques, etc. For instance, a nonuniform preheating of the prefabricated grip or the formation of oxide layers upon the latter will detrimentally affect the fusion between the wall of the parison and this separately fabricated element.
To obviate this disadvantage it has already been proposed (see German Patent No. DT-PS 11 76 829) to form the handgrips directly upon the container by introducing the parison into the blow mold which is provided, at the portions which close against one another to form a common edge or seam, with formations engaging outwardly bulging parts of the parison. The edges thus engage such outwardly bulging parts and shape the latter into the handgrip configuration by welding opposing wall portions together and producing a sectoral recess or cutout adapted to enable the fingers of the hand of the user to pass around the grip.
The disadvantage of this system, in spite of the fact that it eliminates the need to interrupt the container-producing cycle, is that portions of the parison must be introduced into formation by blowing them into the sector-shaped elements provided with these formations. As a consequence, the material of the parison stretches nonuniformly in these regions and the portions of the thermoplastic material tend to cool upon contact with the aforementioned elements so that the blowing is not always effective. This leads to weakening of the container wall in the region of the grips and incompletely formed handgrip members unless special steps are provided to ensure full positioning of the wall portions of the parison in the forming elements. In fact, experience has shown that there was a tendency for the container wall to tear at the region of the handgrips. As a consequence earlier processes of the aforedescribed type have proved to be impractical for mass production.